Because of its excellent characteristics such as high water resistance, high transparency, high strength, good thermoformability and low cost, a molded product of thermoplastic plastic has been heretofore in an enormous amount for many purposes such as agricultural film for heat-retaining, moisture-retaining crops during growth or protecting crops against external factors such as rain and wind, civil engineering sheet for preventing the flow of earth and sand, film and sheet which acts as a packaging material for various foods, beverages, chemicals, miscellaneous goods, etc., packaging bag or lightweight vessel obtained by thermoforming film or sheet, injection-molded vessel and blow-molded vessel.
Examples of thermoplastics which have heretofore been normally used as general-purpose resins include polyethylene, polypropylene, polystyrene, and polyvinyl chloride. After used, these plastic wastes are disposed of by incineration or reclamation. However, these disposal methods are disadvantageous in that they overflow the land to be reclaimed, mar the scene, threaten oceanic life, pollute the environment or cause other global environmental problems.
In particular, when resins such as polyethylene, polypropylene and polystyrene are incinerated, these resins give a high combustion calorie that can damage the furnace and thus shorten the life thereof. On the other hand, when a polyvinyl chloride is incinerated, it gives a low combustion calorie but produces harmful gases. Referring to reclamation, it is known that these general-purpose resins have a high chemical resistance and thus semipermanently remain undegraded and intact, partly causing the shortage of land to be reclaimed.
Further, when these thermoplastic plastics are discarded in the natural environment, they mar the appearance of the nature or can be accidentally ate by marine life, birds, etc. to cause the loss of rare living resources, partly causing the environmental destruction. In order to solve these social problems, biodegradable polymers are now under extensive study.
For example, as a multi-film for the purpose of keeping heat and humidity to help the growth of crops or agricultural film or sheet such as green house sheet for protecting crops against severe external factors there has heretofore been used a polyolefin resin such as polyvinyl chloride, polyethylene and polypropylene. After used, these films or sheets are freed of soil accumulated, collected and then discarded.
This work requires much labor. Further, these waste films or sheets are partly incinerated. However, they are mostly stored or incinerated outdoor. Thus, they can be easily ate by birds. Further, when incinerated they discharge hydrogen chloride gas to the atmosphere, accelerating the envirorunental pollution.
If these films or sheets can be prepared from a biodegradable polymer so that they can degrade and disappear in soil rapidly after used, the time and required to remove them can be eliminated, and the burden on the environment can be lessened.
This applies to civil engineering sheets. At present, polyvinyl chloride sheets are mostly used. If these sheets can withstand working conditions during use but can rapidly degrade and disappear after used, the time and labor required to remove them can be eliminated, and the burden on the environment can be lessened.
This also applies to packaging materials for various liquids, powders and solid matters such as food, beverage, chemical and miscellaneous goods. It has been expected to replace the conventional polymer by a biodegradable polymer which can readily degrade after used to lessen the burden on the environment.
One of these biodegradable polymers which are noted is polylactic acid called lactic acid-based polymer or its copolymer. A lactic acid-based polymer is one of thermoplastic resins. It has characteristics similar to that of general-purpose resins with respect to water resistance, transparency, strength and thermoformability. In addition, it has characteristics which are not possessed by the general-purpose resins. In other words, a lactic acid-based polymer undergoes hydrolysis reaction and complete decomposition by microorganisms, eventually resulting in the decomposition to water and carbon dioxide Further, since a lactic acid-based polymer has a small combustion calorie, it neither damages the furnace nor produces harmful gases when incinerated. Moreover, as the fermentation starting materials of lactic acid there can be used plant resources, giving various excellent advantages such as reduction of dependence on petroleum resources, which are limited in occurrence.
Molded products of such a biodegradable lactic acid-based polymer need to maintain their desired shape and physical properties during their commercial distribution and use but preferably degrade readily after used. Accordingly, it is preferred that the rate of biodegradation of the lactic acid-based polymer should be controlled as much as possible depending on the purpose of the molded product. A published Japanese translation of PCT international publication for patent application No. 4-504731 discloses that lactide or oligomer incorporated in a polylactic acid as a plasticizer accelerates the deterioration of the polylactic acid.
Thus, it can be proposed to increase the content of lactide in the polylactic acid in order to enhance the degradation rate of polylactic acid. However, if lactide is incorporated in a polylactic acid, it is sublimed during thermoforming (extrusion film-forming, extrusion molding or injection molding) to adhere to or deposit on the molding apparatus such as chill roll and cooling mold. Therefore, the resulting deposit mark of lactide is then transferred to the molded product, remarkably marring the quality thereof. At the same time, the incorporation of lactide makes it difficult to form the polylactic acid. Moreover, lactide can leak front the molded product during storage, causing fatal stain on the molded product, particularly in film or sheet form. Such stain not only mar the external appearance of the molded product but also attracts dust to contaminate merchandises contained therein as well.